Difference between gravitational mass and inertial mass

It's a statement of the equivalence principle that an object accelerated in the absence of any gravitational attraction (inertial mass acceleration), will experience events indistiguishable from that produced by an equivalent magnitude gravitational field (gravitational mass acceleration).

The difference is one refers to acceleration produced by any force other than gravity and the other refers to that produced by gravitational attraction.

The classic example is the astronaut on a spacerocket. As the engines accelerate both the craft and the astronaut, a reaction force pressing the astronaut against the craft in the direction of the acceleration will be experienced. To the astronaut, this will be indistinguishable from the apparent acceleration experienced due to gravitational attraction producing the same experience.

It leads to the conclusion that there is no absolute frame of reference for acceleration and is analogous to relativity-theory that there is no absolute frame of reference for velocity.